From a competitive Pokemon standpoint, Alakazam is better. Notably, Alakazam's base 120 Speed allows it to outspeed many Pokemon, including the crowded base 100 benchmark and the already-fast 110 benchmark. Its 135 Special Attack is also 10 greater than Gardevoir's 125. Alakazam's Magic Guard also makes it immune to all sorts of passive damage, most notably entry hazards, Toxic, Sandstorm, and its own Life Orb's recoil damage. Gardevoir does have their strengths; Fairy typing is nice, and having a secondary STAB super effective against Dark is always nice for a Psychic type. However, neither Alakazam nor Gardevoir are great hit-takers, meaning Gardevoir's Fairy typing doesn't offer as much defensive capability as one may imagine, most notably only giving the Dragon immunity. Gardevoir can still be knocked out from common attacks much in the same way Alakazam can be, and this is exacerbated by the fact that Gardevoir is not immune to hazard damage. With Gardevoir's defensive advantage being pretty much null, Alakazam's more powerful Psychic-type attacks become much more appealing, especially since it still has options like Focus Blast and Dazzling Gleam to deal with Dark types.
From a playthrough perspective, it's still Alakazam, but only if you have access to TMs for moves like Dazzling Gleam or Focus Blast. Much of what I said above applies to a playthrough; however, Alakazam only learns Psychic-type attacks through level up, meaning to have any way to hit Dark types, you must invest a TM on it. In the Generation 1 games and their remakes, this would be fine due to the lack of Dark types and only one Steel type evolutionary line. However, take the Gen 3 or Gen 5 games, which both have a whole Dark type Elite 4 member. I believe that for the Gen 3 games (Ruby Sapphire Emerald, not FireRed LeafGreen) Gardevoir is better, as there is no Focus Blast TM, the Dark type Elite 4 member Sidney, the Ghost type Elite 4 member Phoebe that has a Sableye, and several of the gym leaders' Pokemon being Psychic, Steel, or Dark type. However, if you value the neutral coverage of Alakazam thanks to the elemental punches, Alakazam can still work, you just maybe want a Fighting type on your team as well. For every other game, Alakazam with Focus Blast or Dazzling Gleam is going to be better than Gardevoir; however, Gardevoir's Fairy typing is much more useful here than in competitive and may prove useful in certain situations.
